The invention relates to a brewing unit for a coffee machine comprising a brewing cylinder and a piston, the brewing cylinder and the piston being movable in the axial direction relative to one another.
In fully automatic coffee machines the preparation of coffee is done in portions. This requires a brewing unit in which the ground coffee powder is compressed and subsequently brewed. Such brewing units have a brewing chamber formed by a brewing cylinder which is sealed by two pistons at its open ends. At least one of the two pistons and the brewing cylinder are movable relative to one another. The movement of these elements toward one another compresses the coffee powder introduced via a duct into the brewing chamber between the two pistons, and subsequently ejects the leached-out coffee powder. One of the two pistons, has a conduit to supply the hot water required for the brewing process, while an outlet conduit is connected to the other piston. The outlet conduit, in turn, is connected to a coffee delivery conduit to fill the brewed coffee into a drinking vessel, for example a cup. Different methods are known for conceptualizing the necessary movement with respect to one another of the individual elements of a brewing unit.
For example, EP 0 659 377 A1 describes a driving system which moves the brewing chamber with respect to the two pistons. To compress the coffee power, one piston is entrained by the brewing chamber, while the other piston serves as an abutment. Consequently in this brewing unit only two elements are movable with respect to one another, namely the brewing cylinder and one of the two pistons. Also known are brewing units where both pistons and the brewing cylinder are all movable relative to one another and are moved with one another or also toward one another in the course of a brewing cycle.
In all previously known brewing units, each of the pistons includes an encircling sealing ring for sealing off the piston against the inner wall of the brewing cylinder. The sealing rings are in contact with inner wall of the brewing cylinder with sufficient compression such that the necessary seal of the brewing chamber is ensured, even at the higher pressures of the brewing chamber when brewing espresso. In brewing units where coffee is prepared under increased pressure, the piston seal must be in contact with the inner wall of the brewing cylinder under a correspondingly high compression. However, with increased compression the frictional resistance naturally increases during movement of the piston with respect to the brewing cylinder. Consequently, when such sealing rings are employed the motor drive of the movable elements of the brewing unit must be correspondingly strong.
The brewing cylinder comprises a filling duct or a filling opening to allow ground coffee powder to be introduced into the brewing chamber formed by a section of the brewing cylinder and the two pistons. The coffee is brewed in a section of the brewing cylinder spaced apart from the filling opening. The section is formed by the two pistons in the brewing position of the brewing unit. In order to transport the coffee powder into this section of the brewing cylinder, at least one of the two pistons is moved with respect to the brewing cylinder. During this movement the sealing ring of the piston is also guided past the filling opening, so that the section of the sealing ring extending over the opening relaxes in it due to its material elasticity and, upon reaching the edge of the filling opening, in turn, is compressed by the inner wall of the brewing cylinder. Therefore, the sealing ring is exposed to wear at this site. To counteract such wear, EP 0 484 277 B1 proposes to dispose the piston which is moved with respect to the brewing cylinder in a sleeve. The sleeve is transported past the filling opening together with the piston and only after the transport is the piston extended out of the sleeve to compress the coffee powder. While this measure counteracts the one-sided wear of the sealing ring when guiding it past the filling opening, the sealing ring is subjected to uniform wear and tear with respect to its circumference in the process of the retracting and extending from the sleeve since the piston with the sealing ring must be extended out of the sleeve and retracted again.
The foregoing example of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.
Building on this discussed prior art, the invention therefore has as its aim to propose a brewing unit for a coffee machine, in which the problems discussed in connection with the addressed prior art with respect to the necessary tightness of the brewing chamber during the brewing, on the one hand, and the wear of the piston seal, on the other hand, is solved.